1. Field of the Invention
This invention relates to external lighting devices for use with still cameras, and more particularly to a method and apparatus for coupling a remote light source to a still camera and for activating and preferably modulating the remote light source.
2. Description of Related Art
Still cameras are frequently used with artificial light sources, such as flash or strobe units, in order to provide more or different light than is naturally available. In most amateur photography, the photographer uses a single strobe that is directly attached to a camera. However, there are many situations in which photographers use one or more strobes that are physically remote from the camera. Studio photographers frequently use several highpowered strobes positioned at remote locations with respect to the camera in order to provide a desired direction, color, and quality of light to the subject. Similar techniques are used by photographers who are photographing large or distant outdoor subjects. Furthermore, because of the light filtering qualities of water, it is customary to use remote strobes in underwater photography. Underwater strobes are almost universally sealed in a watertight enclosure that is separate and apart from the enclosure in which the associated camera is sealed. In all these situations, a communication link must be established between the camera and each associated strobe in order to synchronize the triggering of the strobe to the activation of the shutter of the associated camera.
Communication between a camera and remote strobe is commonly provided by a multi-conductor electrical cable having an electrical connector at each end. Such a cable is commonly referred to as a "SYNC" cord. FIG. 1 is an illustration of a conventional camera 101 and a strobe 102 connected by a conventional SYNC cord 107. One end of the SYNC cord 107 mates with an electrical connector 103 provided on the housing of the camera 101. The other end of the SYNC cord 107 mates with an electrical connector 105 provided on the housing of the strobe 102. Some prior art strobes are provided with a SYNC cord having only one connector at the distal end for connecting to the housing of a camera. The other end is fixed to the strobe (e.g., attached by solder).
In accordance with one prior art scheme for triggering a remote strobe, the SYNC cord 107 has two wires. The first wire conducts a SYNC signal. The second wire serves as a GROUND return path. The SYNC signal is commonly controlled using an "open collector" protocol. The open collector output for the SYNC signal is "pulled-up" by a resistor coupled to a voltage source. The SYNC signal is shorted to ground to trigger the strobe. This simple scheme allows a mechanical switch to trigger a strobe. The mechanical switch may be mechanically coupled to the shutter of the camera. This scheme also allows the low-cost analog signal processing in the strobe. Furthermore, this scheme allows a plurality of strobes to be connected in parallel and triggered at once. Still further, the propagation delay between initiation of the command to trigger the strobe and receipt of the command at the strobe which results from this scheme is very small (on the order of 3 to 4 microseconds).
One problem with the above described system is that while the camera 101 controls the time at which the strobe 102 is triggered, there is no control from the camera over the amount of light the strobe 102 generates. In the mid-1980's, a system was introduced for triggering remote strobes. In accordance with this new system, the strobe is turned off when the film plane has accumulated an appropriate amount of light (as determined by the type of film used and the settings on the camera). Typically, a photo-sensor (photo-integrator) built into the camera is focused on the film plane to determine the amount of light received. When sufficient light has been received on the film plane, a stop signal is sent from the camera to the strobe, causing the strobe to turn off. Such a system is commonly referred to as a TTL ("through the lens") exposure control system. TTL exposure control systems require a STOP wire in addition to the GROUND wire and SYNC wires discussed above. The STOP signal is transmitted by an open collector protocol similar to the protocol used with the SYNC signal. Use of an open collector STOP signal allows multiple strobes to be controlled by the same STOP signal. As noted above; such open collector control has relatively short propagation delay when the STOP wire is shorted to the GROUND wire. Using a third separate wire to communicate the STOP signal provides full backward compatibility with two wire "non-TTL" cameras. In addition, there are significant manufacturing economies which result from isolating the TIL exposure control functionality from the SYNC functionality, since the camera or strobe may easily be made in two versions: one which includes TTL functionality, and the other which does not.
Many cameras currently use a protocol which also includes a READY signal sent from the strobe 102 to the camera 101. The READY signal permits the camera 101 (and the photographer, if the camera has a suitable display in its view finder) to know whether the strobe 102 is ready to be triggered. Unlike the SYNC and STOP signals, which are edge-triggered, the READY signal is a simple active-high logic signal. That is, the strobe 102 pulls the READY line high when it is in the "READY" state (i.e., the strobe is fully charged and ready to be triggered), and leaves it low when it is not ready (i.e., it is either off or is not fully charged).
In situations in which a photographer is using multiple strobes attached to one camera, the READY signal from only one strobe is transmitted to the camera. Accordingly, information regarding the status of the other strobes is not available to the camera. Therefore, the photographer must look at each other strobe to determine whether those strobes are ready to be triggered.
A fifth signal is also commonly used in cameras having TTL exposure control. The fifth (MONITOR) signal is an active-high current signal (as opposed to a voltage signal). The MONITOR signal indicates to the strobe that the camera is "awake" and that a pulse on the SYNC signal may be forthcoming. Typically, a camera begins transmitting the MONITOR signal when the photographer has partially depressed the shutter button. The camera continues transmitting the MONITOR signal for some period of time afterwards (usually between 10 seconds and 1 minute).
One problem that exists with prior art systems is the lack of a mechanical standard for interfacing a SYNC cord with a camera and remote light source. While the connectors on the camera side of a SYNC cord are reasonably standard, each manufacturer uses its own mechanically different connector to attach the SYNC cord to a remote light source. Because each manufacturer has a different mechanical standard, different SYNC cords are required for use with different manufacturers' strobes.
Even on the camera side, the standardization of the mechanical interface is not complete. Some camera manufacturers use a unique SYNC cord mechanical interface. Therefore, different SYNC cords may be required for different cameras as well. This lack of standardization makes it necessary for a photographer who owns strobes from more than one manufacturer to purchase at least one SYNC cord for each type of strobe he owns. This represents a considerable expense, since each SYNC cord is relatively expensive (e.g., approximately $75.00 each for underwater SYNC cords, and as much as $100 to $200 for above-water TTL sync cords). Furthermore, a photographer who owns a camera from a manufacturer that uses a non-standard interface is generally forced to purchase strobes made only by that manufacturer. Still further, above-water TTL SYNC cords are available in relatively short lengths (up to about 10 feet) and are quite bulky due to the heavy shielding necessary to prevent false-triggering of the strobe's STOP circuit by the pulse of electromagnetic noise that is created when a strobe is triggered. Still further, a photographer who wishes to use more than one strobe (particularly in an underwater setting) must purchase a "dual" SYNC cord in order to interface one camera to two strobes. While most underwater strobe manufacturers make "dual" SYNC cords (which are even more expensive than single cords), all manufacturers use their own connectors on the strobe ends of the dual SYNC cords they sell, making it difficult, and in most cases impractical, to combine strobes from different manufacturers in a single system. Using more than two strobes underwater requires additional, specialized hardware, since manufacturers of underwater equipment do not offer triple SYNC cords as a standard product. Above water, similar problems are encountered. Very few, if any, commercial products exist which permit the photographer to use TTL exposure control with multiple remote strobes in an above-water setting. Instead, the photographer is forced to rely on SYNC-only triggering of multiple strobes, either through two-wire electrical connections or by using the strobes in "slave" mode, as described below.
One common way to address the problem of using multiple strobes is to have one strobe attached to a camera via a single SYNC cord and to have a second strobe triggered in "slave" mode. In slave mode, a photo-sensor in the second strobe is used to detect a burst of light emitted from the first strobe. The photo-sensor then triggers the second strobe in response to detection of the burst of light. By operating strobes from different manufacturers in slave mode, the difficulties which result from each manufacturer having a different mechanical interface is eliminated. In addition, this system allows a relatively large number of strobes to be used. However, slave triggering of a strobe is notoriously unreliable, both with regard to false triggering and failure to trigger, since the illumination of a photo-sensor in the second strobe is typically indirect (e.g., both strobes are typically pointing at the subject). In addition, few if any commercially available strobes permit TTL exposure control to be used in slave mode.
Another disadvantage of prior art systems is the reliability of the SYNC cord. That is, the reliability of the system will be compromised by any of a number of mechanical failures, all of which occur relatively frequently. In underwater systems, a common problem is the breach of the watertight seal between the cable and the strobe or camera. Failure of a watertight seal in an underwater camera or strobe will likely result in the destruction of either the camera, the strobe, or both. Well-designed watertight seals typically have a high reliability if the SYNC cord is not removed from the camera and the strobe frequently. Unfortunately, in practice, a photographer frequently disconnects the SYNC cord from the camera and the strobe for ease of transportation, and to change camera and strobe configurations. In the case of both underwater and outdoor photographers, this is often done in a moist environment, thus decreasing the reliability of the electrical connections between the camera and strobe. Each time the connection between the SYNC cord and either the camera or strobe is broken, particularly in a moist environment, there is a chance that the electrical connection between the pins and sockets that make up the individual contacts of the connectors will become corroded, resulting in electrical failure of the SYNC cord. In addition, conventional SYNC cords are susceptible to breakage of the wire inside the cable due to stress of use. Furthermore, flooding of the cable itself (through the insulating jacket) due to wear or misuse is common in underwater applications.
A further disadvantage of prior art systems is inability to reconfigure the system underwater or in harsh environments. Any attempt to disconnect a SYNC cord from either the strobe or camera underwater would breach the integrity of the watertight seals in the system. Furthermore, conventional SYNC cords are relatively bulky and heavy, making transportation and storage inconvenient, particularly in the case in which several SYNC cords are required.
Still further, conventional SYNC cords are of a relatively short and fixed length and cannot be adjusted or made to a specific length in the field without soldering or crimping electrical conductors to pins and/or sockets of the connector.
Still further, conventional SYNC cords are expensive to manufacture and purchase. The connectors at either end of the SYNC cord typically consist of many small pins and/or sockets which must be soldered or crimped to the ends of the individual wires, and then inserted into a metal connector body which is overmolded onto the end of the cable. Particularly for above-water sync cords, the cable used for this purpose must be heavily shielded to ensure proper operation ofthe TTL exposure control in spite of the electromagnetic emissions caused by the discharge of the strobe's capacitors. In addition to the expense of the SYNC cords themselves, the inclusion of the mating connector on the camera and strobe adds to the expense ofthe overall system. In fact, in order to reduce costs, manufacturers sometimes omit the connector on the strobe side, thus sacrificing convenience and reliability to reduce costs.
FIG. 2 is an illustration of one system available today in which the problem of connecting and disconnecting SYNC cords underwater is overcome by providing a SYNC cord 200 having optical linkage modules 201, 203. For example, one product has an optical transmitter unit 201 on one end of a first long cable 205. The other end of the first cable 205 is attached to a camera 101 using the conventional electrical connector 103 provided on the camera 101. A receiver unit 203 is coupled by a long electrical cable 207 to the connector 105 for making electrical connections to a strobe. The first and second cables 205, 207 are coupled together by directly interfacing the optical receiver unit 203 and transmitter unit 201. Alternatively, the optical receiver unit 203 may be coupled to two long cables for connecting to two strobes. These cables 205, 207 are heavy and bulky. The transmitter unit 201 and receiver unit 203 each have two separate optical transmitter/receiver pairs, one dedicated to transmitting the SYNC signal and the other dedicated to transmitting the STOP signal. Each channel consists of an LED (light emitting diode) and a photo-detector. The LED and photo-detector have optically isolated line-of-sight contact through transparent windows in the transmitter unit 201 and receiver unit 203, respectively. This system does not address many of the reliability issues associated with the use of electrical cables underwater. Further, it relies on the surrounding water and physical distance of the optical modules from the strobe to shield the modules from unwanted electromagnetic interference.
Another system for use underwater is disclosed in Japanese Publication No. JP 55036829 filed in the name of Mamiya Koki Kk (the "Mamiya" patent). The Mamiya patent discloses an underwater camera 1 and underwater strobe 2 connected by a light guide 3, as shown in FIG. 3. A watertight light-emitting window 7 integrated into the camera directs light through an optical connector 4 into the light guide 3 when the light guide 3 is coupled to the camera 1. Connectors 4 are fixed at each end of the light guide to allow the light guide 3 to be coupled to the camera 1 and strobe 2. A light-receiving element 10 integrated into the strobe 2 receives the light which traverses the light guide 3. Upon receiving the light emitted by the camera 1 and transmitted through the connectors 4 and light guide 3, the strobe 2 is triggered. Since the connection between the light guide 3 and the camera 1 and between the light guide 3 and the strobe 2 are water tight, the camera 1 and strobe 2 can be disconnected from the light guide 3 while underwater. However, this system can only be used with cameras and strobes that have the watertight optical connectors 4 required to interface with the light guide 3. Furthermore, the use of connectors 4 on either end of the light guide 3 fixes the length of the light guide 3 at the time of manufacture. In addition, this system does not provide any way for multiple strobes to be used. Still further, this system does not permit TTL exposure control.
Accordingly, there is a need for a SYNC cord that can be used to transmit TTL signals between a camera and strobe in a manner which allows the camera and strobe system to be reconfigured (e.g., allows the SYNC cord to be disconnected from the camera and strobe underwater or in harsh environments). Furthermore, there is a need for a small lightweight SYNC cord for coupling TTL signals between a camera and strobe. Still further, there is a need for a more reliable SYNC cord. Still further, there is a need for a SYNC cord which has the above-mentioned features and which does not require any change in the design or manufacture of conventional cameras and strobes. Furthermore, there is a need for a SYNC cord which permits convenient simultaneous use of strobes from different manufacturers. Still further, there is a need for a system which allows a relatively large number of strobes to be reliably triggered by a single camera. Furthermore, there is a need for an inexpensive SYNC cord for transmitting TTL signals between a camera and a strobe. Still further, there is a need for a SYNC cord which can be adjusted in length by the user in the field. Furthermore, there is a need for a TTL sync cord with a greater maximum usable length. The present invention provides a SYNC cord which satisfies these needs.